Etendue-Preserving Raman Sampling Optics

The present application is a divisional application of U.S. patent application Ser. No. 18/148,519, filed on Dec. 30, 2022 of which the entire disclosure of the pending, prior application is hereby incorporated by reference.

The present disclosure relates generally to spectroscopy and, in particular, to Raman spectroscopy and, more particularly, to Raman analysis systems incorporating micro-lens and/or micro-mirror arrays for improved collection efficiency.

Induced radiation effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular composition. A conventional Raman analysis system generally includes three main components: a laser excitation source, sampling optics and a spectrometer. Because Raman instruments use lasers in the visible to near-infrared region of the electromagnetic spectrum, optical fibers can be used to carry the laser excitation and collect the scattered radiation from the sample. In process control and other applications, an optical probe, e.g., a Raman probe, can be inserted into a reaction or used to collect Raman spectra though a window, for example, in an external reaction sample loop or flow cell, thereby eliminating sample contamination.

1 FIG. 100 100 102 104 108 106 120 122 124 100 101 101 122 120 122 116 114 112 is a schematic diagram of a conventional fiber-based Raman probe. Excitation illumination is supplied to the probeover fiber, which is then collimated by lens. The collimated light then passes through a bandpass filterto remove non-laser wavelengths. The filtered light is reflected by a mirroronto a beam combiner, which is then directed to a sample along a counter-propagating collimated path. An objective elementof the probefocuses the beam of excitation illumination to a point on or in a sample mediumand recollimates scattered light from the sampleback into path, through beam combiner. The combined beammay be filtered by a notch filterto remove a portion of the scattered light having the same wavelength as the laser (e.g., the laser line) before being focused by lensonto the end of a collection fiber.

2 FIG. 1 FIG. 202 204 208 206 122 206 is a simplified block diagram of a conventional flow cell for Raman analysis. The laser is represented by block, and the spectrometer by block. A computermay be provided to control system operation, to provide for a user interface and to receive and analyze Raman signals separated by the spectrometer, for example. Blockrepresents beam-combining optics to generate the collimated, counter-propagating, combined excitation-collection path. Blockmay be fiber-coupled probe like the one depicted in, with the understanding that other probe configurations are possible, including direct-coupled (e.g., non-fiber) designs.

2 FIG. 122 212 214 216 216 220 122 214 212 122 In, the combined beamis focused by objective lensto a pointin a conduit, which contains a sample to be analyzed. The conduitmay be a primary flow tube, process vessel, or a capillary branch from a primary tube or vessel. A reflectormay be provided to achieve a ‘multi-pass’ configuration, by which the combined beampasses through the point, returning to the objective lens. While such an arrangement generates additional signal through relayed imaging, it still does not increase the solid angle of the combined beam.

1 FIG. 122 124 Etendue is often referred to in relation to how ‘spread out’ the light in an optical system is, or conversely, the maximum ‘concentration’ which can be achieved under condensing conditions. Because etendue is the integrated product of the emitter area and the solid angle, in, the solid angle of beamis assumed to be constant between the individual, optical components and the single focusing lens, such that a simple spot-area summation is sufficient to define the effect.

While single-lens Raman probe objectives are well-known and readily manufactured, they have drawbacks, including strict reliance on a single point or isolated sampling region. If the sample spot is temporarily or permanently disrupted for any reason, creating inconsistencies associated with flow or mixing, the target may not be an accurate representation of sample composition. As such, the need remains for optical geometries to improve signal-generation capabilities in Raman flow-cell and other signal collection configurations while, ideally, preserving system etendue.

In one aspect of the present disclosure, sampling optics for a spectroscopic system incorporating a collimated beam of light, including an excitation beam and a counter-propagating collection beam, comprise: an objective optical element comprising an array of closely packed optical elements, each array optical element configured to focus a portion of the collimated beam onto or into a sample and to recollimate a portion of light received at the objective optical element from the sample such that the objective optical element is operative to focus the collimated beam onto or into the sample and to recollimate the light received from the sample, wherein a system etendue is defined with respect to the use of a single objective element, and wherein the array of closely packed optical elements is configured such that etendue contributed by the individual array optical elements yields a combined etendue that is substantially similar to the system etendue. The portions of light received from the sample and recollimated by the individual array optical elements are integrated and averaged to form the collection beam.

In at least one embodiment, the array optical elements comprise spherical surfaces. In certain embodiments, the array optical elements comprise aspherical surfaces. In further embodiments, the array of closely packed optical elements is an array of closely packed lenses or transmissive elements. In still further embodiments, the array of closely packed optical elements is an array of closely packed mirrors or reflective elements.

In at least one embodiment, the array of closely packed optical elements is an array of closely packed lenses or transmissive elements, each having an optical axis, and the sample optics further comprise an array of closely packed mirrors or reflective elements, each having an optical axis that is on-axis with a respective one of the lenses or transmissive elements of the array of closely packed optical elements. In such an embodiment, the closely packed lenses or transmissive elements may be disposed on one side of a sample volume, and the closely packed mirrors or reflective elements may be disposed on an opposing side of the sample volume. In such an embodiment, the sample volume may be within a flow cell.

In certain embodiments, the closely packed optical elements are arranged in one of the following array configurations: hexagonal, linear and radial. In further embodiments, the array of closely packed optical elements is configured as an integrated panel. In such an embodiment, the panel may be flat or curved.

In at least one embodiment, the sampling optics further comprising one or more amplifying optical elements configured to amplify the light received from the sample. In such an embodiment, the one or more amplifying optical elements may comprise an array of closely packed mirrors or reflective elements configured such that the excitation and collection beams make multiple passes through the sample.

In at least one embodiment, the objective optical element comprising the array of closely packed optical elements is integrated into a remote optical measurement probe. In further embodiments, the objective optical element comprising the array of closely packed optical elements is integrated into a flow cell.

In a further aspect of the present disclosure, a Raman measurement probe comprises: an optical input configured to receive an excitation light beam from a laser; an optical output configured to convey a collection light beam to a spectrometer; a beam-combining optical element operative to merge the excitation light beam and the collection light beam into a collimated, counter-propagating excitation-collection light beam; and an objective optical element comprising an array of closely packed optical elements, each array optical element configured to focus a portion of the collimated beam onto or into a sample and to recollimate a portion of light received at the objective optical element from the sample such that the objective optical element is operative to focus the collimated beam onto or into the sample and to recollimate the light received from the sample. In such an embodiment, a system etendue is defined with respect to the use of a single objective element, and wherein the array of closely packed optical elements may be configured such that etendue contributed by the individual array optical elements yields a combined etendue that is substantially similar to the system etendue. In at least one embodiment, the Raman measurement probe further comprises: a first optical fiber configured to convey the excitation light beam from the laser to the optical input; and a second optical fiber configured to convey the collection light from the optical output to the spectrometer.

In another aspect of the present disclosure, a Raman flow cell comprises: a conduit configured to convey a sample, the conduit including at least one sidewall transparent to wavelengths associated with Raman measurement; an objective optical element operative to focus a collimated, counter-propagating laser excitation and Raman collection light beam into the sample through the at least one sidewall and to recollimate light received from the sample through the at least one sidewall, wherein the objective optical element comprises an array of closely packed optical elements, each array optical element configured to focus a portion of the collimated beam onto or into a sample and to recollimate a portion of light received at the objective optical element from the sample. The sample is a liquid or a gas.

In at least one embodiment, the array of closely packed optical elements is an array of closely packed lenses or transmissive elements, each having an optical axis, and the flow cell further comprises an array of closely packed mirrors or reflective elements, each having an optical axis that is on-axis with a respective one of the lenses or transmissive elements of the array of closely packed optical elements. In such an embodiment, the array of closely packed mirrors or reflective elements may be immersed within the sample. IN a further embodiment, the at least one transparent sidewall extends to opposing sides of the conduit, and the array of closely packed lenses or transmissive elements and the array closely packed mirrors or reflective elements are disposed on opposing sides of the conduit, such that each corresponding pair of lenses or transmissive elements and mirrors or reflective elements focus the excitation-collection beam within the sample.

In yet another aspect of the present disclosure, in a Raman analysis system characterized in having a system etendue, and wherein a combined excitation-collection beam is directed to and from a sample, an improvement is disclosed, comprising: sampling optics incorporating an array of lenses or mirrors, each lens or mirror of the array configured to: focus a portion of the combined excitation-collection beam to a point or region of the sample; and recollimate light scattered by the point or region of the sample and to convey the recollimated light to a spectrometer, wherein the sampling optics are configured and integrated in the Raman analysis system such that substantially preserves the system etendue. In such an embodiment, the array of lenses or mirrors may be disposed adjacent to or within a sample volume of the sample.

124 1 FIG. In broad and general terms, the present disclosure describes systems and methods that improve upon conventional Raman analysis systems by providing sampling optics in the form of an array of optical elements, as opposed to the single objective element used in conventional systems (e.g., the objective elementof). The use of an array of optical elements offers several advantages, including enhanced collection efficiency in conjunction with an inherent integration of collected signals for improved measurement uniformity. Moreover, the etendue using an array of optical elements is substantially the same as the etendue of the system using a single lens. The excitation and collection signals are spatially separated, as defined by the array, such that the etendue associated with each element is fractionally smaller, but when summed, substantially equal the total system's etendue.

In a Raman analysis system incorporating a collimated beam of light including an excitation beam and a counter-propagating collection beam, sampling optics according to the present disclosure includes an objective implemented as an array of closely packed optical elements, each optical element being operative to focus a portion of the collimated beam onto or into a sample and to recollimate a portion of the light received from the sample (e.g., scattered light), and wherein the etendue contributed by the optical elements results in a combined etendue that is substantially similar to the system etendue. As such, the contributions of the light recollimated by the individual optical elements are integrated and averaged to form the collection beam.

According to the present disclosure, the array of closely packed optical elements have spherical or aspherical surfaces. In an embodiment, the array of closely packed optical elements is an array of closely packed lenses or transmissive elements. However, in alternative embodiments, the array of closely packed optical elements may comprise an array of closely packed mirrors or reflective elements. In further embodiments, arrays of lenses and reflective elements may be used together on opposing sides in flow cell configurations and/or for signal amplification involving multiple passes through a sample medium. The array of closely packed mirrors or reflective elements may be on-axis with a respective one of the lenses or transmissive elements.

In embodiments of the present disclosure, the closely packed optical elements may be arranged in hexagonal, linear, radial or other packing geometries. The array may be implemented as an integrated panel, which may be flat or curved. Such panels may be constructed of glass or plastic materials, and the panels may be molded and/or micromachined. The plurality of optical elements may form part of a remote optical measurement probe or may be incorporated into a flow cell. While described in terms of Raman analysis, the systems and methods described herein may be used in conjunction with fluorescent measurement systems with appropriate engineering modification.

1 2 FIGS.and 3 FIG. 1 FIG. 320 304 300 122 Having discussed certain background considerations with respect to,shows a cross-section of an embodiment of an arrayof plano-convex (PCX) lensesaccording to the present disclosure. Numerical referencemay represent the collimated excitation-collection beam exiting a probe, such as the beamshown in, with the understanding that the embodiments described herein are not limited to any particular probe design and, indeed, may be used in conjunction with ‘direct-coupled’ Raman spectroscopic systems, including arrangements without fiber coupling.

3 FIG. 300 320 304 300 306 308 304 320 308 As shown in, the excitation-collection beamimpinges upon the arrayof multiple convex lenses, each focusing a portion of the beamto a pointin a sample medium. As discussed in further detail below, the lens surfacesmay be spherical or aspherical and, in certain embodiments, the arraymay include other optical elements. The sample mediummay be solid, liquid, semi-liquid (such as slurries) or gaseous and the present disclosure may be used in any application, implementation or configuration that a conventional, single objective is used, including flow cells, in situ process monitoring and control.

124 320 304 304 320 102 304 1 FIG. 10 11 FIGS.and As with the single element objectiveof, the arrayof lensesis configured to cover the aperture of the counter-propagating excitation-collection beam in its entirety, as depicted in. As such, the focusing and recollimation functions of each lensare spatially separated, as defined herein by the micro-lens array. In the case of a relayed fiber image into sample space, for example, the fiber, each lensis imaging only a small portion of the fiber object, distributing the object into an array of image slices. Each lens's individual etendue is therefore fractionally smaller, but when summed, the individual etendues equal a total etendue for the system. As a consequence, the total etendue is preserved while affording improved excitation and collection characteristics, including inherent signal integration and averaging.

304 306 306 320 304 In an embodiment, all of the lens elementshave the same focal length, such that the pointsform a plane within the sample volume. Nonetheless, a plane of sample pointsor regions need not be flat, as the arraymay be curved to conform with a particular shape of form of the sample, such as pharmaceutical tables, physiological tissue, and the like. Indeed, in further embodiments, the foci of the lensmay be engineered to focus to a defined surface (either physical or numerical) as opposed to a plane.

304 320 320 As discussed herein, the lensesof the arraymay be spherical or aspheric, and the arraymay be fabricated with any suitable technique using any suitable materials, including glass, polymers and combinations thereof, with or without surface coatings. Manufacturing techniques may include molding, stamping, micromachining, each with or without polishing. The choice of manufacturing technique and materials are to be selected in accordance with desired collection efficiency, sample index matching and other engineering considerations known to those of skill in the art.

3 FIG. 10 11 FIGS.and 3 FIG. 304 320 304 304 304 While not evident in, the lens elementsextend into and/or out of the page, forming a two-dimensional array as better seen in the on-axis views of. Further, while a transmissive panel with a two-dimensional arrayof convex surfacesis shown in, more broadly the array may be refractive, diffractive, or reflective. In such embodiment, the elementsmay be micro-mirrors. As mentioned, spherical as well as aspherical surfaces may be used, as well as Fresnel and holographic implementations of the elements.

304 320 306 The selection of spherical versus aspherical surfacesfor the arraymay be used to adjust the size of the focus regions in the sample, from pointsto more spread areas. The use of aspherical solutions, in particular, may be configured for near-diffraction-limited performance in certain embodiments.

304 320 300 308 304 320 304 308 306 320 In operation, each optical elementof the arrayacts as an independent objective, focusing the combined excitation-collection beaminto the sample. The numerical apertures of each elementmay be comparable to conventional, single-element objectives, but with certain advantages over standard objectives. For example, one distinct advantage is that the arrayof elementsinherently averages the sample interrogation over a larger spatial region of the sample. This averaging effect maintains etendue characteristics of a single-focus solution by spatially integrating multiple, smaller spots. Moreover, the use of a lens arrayas opposed to a single objective may also prove less sensitive to inhomogeneities in the sample such as bubbles, surface imperfections, and the like.

3 FIG. 310 306 304 The Raman effect is inherently polychromatic. Due to this nature, optical systems involved in collecting the Raman scattered light must take into effect the chromatic aberration involved as light propagates through any medium. All transmissive optical components exhibit chromatic effects through refractive index variation. Reflective optics, on the other hand, direct light by reflection, which aside from chromaticity in the reflection coefficient, operate achromatically to focus light. Additionally, while more sensitive to angular alignment, reflective optical solutions typically produce lower background signatures in Raman analysis since the light is not transmitting through a bulk material. Whereasshows divergent conesafter the sample points, in alternative embodiments, an array of mirrors or reflective surfaces, either spherical or aspheric, that focus into the sample may be substituted for the lenses. Such embodiments are more achromatic and exhibit lower background noise.

4 FIG. 402 is a cross-section of an embodiment of the present disclosure incorporating an array of staggered lenses. Such embodiments may facilitate a variation of spatially offset Raman spectroscopy (SORS), enabling sub-surface chemical analysis through tissue, coatings, containers and the like. Whereas conventional SORS uses at least two Raman measurements, one at the source and one at an offset position (e.g., a few millimeters away), with the two spectra can being subtracted, the staggered micro-lens arrays of the present disclosure enable obtaining source and offset measurements simultaneously, using software to perform the subtraction operation.

5 FIG. 500 506 502 506 502 The present disclosure also does not preclude the use of signal amplification techniques, which may include any additional optical element or surface used to retro-reflect and/or recollimated the excitation-collection beam to achieve multiple passes through sample foci. As one example,shows an embodiment of the present disclosure including a planar mirrorpositioned and aligned proximate to a plane of focal pointsproduced by lens arrayto reflect the excitation-collection beam back through the focifor recollimation by lens array.

6 FIG. 608 606 602 602 610 608 612 614 shows an alternative embodiment in which a mirror arrayis positioned and aligned to re-focus the divergent beams passing through focifor re-collimation by lens array. In all embodiments that use lens and reflector combinations, each lens of the arrayis on-axis (depicted by axis) with a corresponding reflector of the mirror array. The reflective surfaces may be mirrored, as shown, or may comprise transmissive surfaces and a flat reflectorconfigured to redirect the light back through the transmissive surfaces as shown at.

7 FIG. 8 FIG. 9 FIG. 702 700 802 900 902 900 908 According to the present disclosure, optical elements such as mirror arrays may also be used for signal amplification and may be placed within a sample volume without the use of micro-lens arrays, as might be the case with certain flow cell implementations. As nonlimiting examples:depicts the use of spherical reflectorsimmersed in a sample volume;illustrates the use of the aspheric reflectors; andshows a pass-through, transmissive panelwith aspheric reflective surfaces. The material used to form the transmissive paneland the aspheric surface geometry of the aspheric reflectorsmay be adjusted for improved sample index-matching.

10 FIG. 11 FIG. As discussed, the lens and mirror arrays of the present disclosure may be fabricated with any suitable technology, and in any lateral arrangement including, without limitation, hexagonal close-packed, linear, radial and asymmetric configurations. As two examples,is an on-axis view of a hexagonal array of lens elements, andis an on-axis view of a linear packing arrangement.